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Overview of Rare Earth Element Investigations inAcid Waters of U. S. Geological SurveyAbandoned Mine Lands Watersheds

By Philip L. Verplanck, D. Kirk Nordstrom, and Howard E. Taylor

ABSTRACT

The geochemistry of rare earth element (REE) variations in acid waters is being studied as part of

the U. S. Geological Survey Abandoned Mine Lands Initiative in two pilot watersheds, upper Animas,

Colorado and Boulder, Montana. The following objectives are under investigation: (1) comparison of

acid mine waters and naturally acidic springs, (2) determination of whether the dominant control on

REEs in acid waters is source-related or post-dissolution process-related, (3) determination of the role of

iron and aluminum colloid formation on the REE patterns, (4) address the utility of REE geochemistry in

acid waters as an analogue for the actinides, and (5) produce a Standard Reference Water Sample forREEs. Results demonstrate that the REE concentrations in acid waters increase with decreasing pH but

tend to be two to three orders of magnitude lower than ore elements such as Cu and Zn. REE patterns are

generally convex-up for waters in the upper Animas, and they are nearly flat with a negative europium

anomalies for waters in the Boulder basin. These results reflect predominantly source-related signatures.

Natural acid springs are frequently, but not consistently, characterized by a negative Ce anomaly that

may be process-related. Field and laboratory experiments indicate that dissolved REEs are affected by

iron and aluminum colloid formation but sorption or coprecipitation with aluminum at pH values greater

than 4.5 is stronger than with iron. Uranium and thorium, however, show a tendency to be removed from

solution more strongly at lower pH (3-4) values, consistent with expected differences in oxidation state

and a stronger affinity for iron precipitation.

INTRODUCTION

Rare earth element (REE) geochemistry is a

powerful tool for identifying geochemical

processes (Brookins, 1989). This has been

demonstrated in many petrologic studies but is

just beginning to be applied to aqueous systems.

REEs have been used as geochemical tracers

because of their unique, coherent chemical

behavior. The REEs are a suite of fourteen metals

from atomic number 57 (La) to 71 (Lu) that have

similar chemical and physical properties. There

are, however, small differences in geochemical

behavior because with increasing atomic number

there is a systematic decrease in ionic radius. The

REEs are trivalent with the exception of Ce (also

4+) and Eu (also 2+); therefore, the behavior of

Ce and Eu relative to the other REEs can

potentially be used as a probe of redox conditions

of an environmental system (Loveland, 1989).

Elucidation of the geochemical behavior of

REEs in a weathering environment has been

hindered by the very low aqueous concentrations,

which generally are less than one microgram per

liter (g/L) in surface and ground waters.With

the advent of inductively coupled plasma-mass

spectrometry (ICP-MS) the determination of REE

concentrations in waters has become more

routine. Concentrations of REEs are usually

normalized to a reference standard, such as

chondrite or North American Shale Composite

(NASC), or to a sample of interest. By

normalizing the REE concentrations, the

characteristic zigzag pattern due to the increased

stability of the even masses is eliminated, and


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subtle variations in the REE pattern can be

recognized.

Recent studies have demonstrated the use

of REE geochemistry in the interpretation of

water-rock interactions (Smedley, 1991; Fee and

others, 1992; Johannesson and others, 1997).

Relatively few studies have investigated the

behavior of REEs in an acidic weatheringenvironment (Auque and others, 1993, 1994;

Johannesson and others, 1994; Johannesson and

Lyons, 1995; Gimeno and others, 1996;

Johannesson and others, 1996), and none of these

studies have sampled mined areas. Previous

investigations have revealed a general decrease in

REE concentrations with pH increase, a

characteristic convex-up NASC-normalized

pattern, and no consistency with respect to Ce

anomalies. Interpretations focus on whether these

features are source-related (Smedley, 1991;

Sholkovitz, 1995) or process-related (Sholkovitz,

1995; Johannesson and others, 1996; Byrne and

Sholkovitz, 1996). One of the main interests has

been the effect of iron and aluminum colloids on

REEs in rivers, estuaries, and seawater, but there

has been no direct study of the effect of colloid

formation on REE fractionation between aqueous

phase and colloidal phase.

As with most elements, the REE

concentrations of stream waters may be

controlled by water-rock processes along the

subsurface flow path as well as the in-streamenvironment. These processes include dissolution

and precipitation of minerals, oxidation and

reduction reactions, and adsorption and

desorption reactions with secondary minerals or

colloidal particles. In most igneous rocks, the

dominant rock type in the study areas, the REEs

primarily occur within accessory phases,

including apatite, zircon, monazite, allanite,

titanite, and epidote. Release of the REEs from

these minerals is complex owing to the

occurrence of accessory phases as inclusions in

major mineral phases. Also, some accessoryphases are extremely resistant to weathering.

Once released from the primary mineral phase,

REEs may be sequestered by secondary mineral

phases. Detailed mineralogical data including

mineral occurrences, compositions, and

morphology are needed to unravel this aspect of

acidic weathering environments. This part of the

study is in progress and will not be discussed in

this overview.

The U. S. Geological Survey (USGS)

Abandoned Mine Lands (AML) watersheds are

well-suited to investigate the many processes that

potentially control the REE geochemistry of acid

waters because of the numerous acid water

sources and the interdisciplinary approach towatershed characterization. REE geochemistry is

being used to try to differentiate between natural

and anthropogenic sources of acid waters and

metals, as well as to determine processes

controlling the fate and transport of metals

entering the fluvial system.

This paper is an overview of our

investigations into the REE geochemistry of the

acidic weathering environment, including water-

rock interaction and in-stream processes. Results

from field and laboratory investigations are

reported. In addition, two new Standard

Reference Water Samples were produced to

evaluate and control analytical measurements.

Such a reference sample has not previously been

available.

METHODS

Spring, stream and mine water samples

were collected during low flow in the AML pilot

watersheds, the upper Animas River basin, Colo.

and the Boulder basin, Mont. Water temperature,pH, specific conductance, and Eh were

determined on site. The Eh and pH were

measured by placing electrodes in a flow-

through-cell through which the sample was

pumped with a portable peristaltic pump (Ball and

others, 1976). The pH electrode was calibrated on

site with pH buffers, 1.68, 4.01, 7.00, and 10.00,

that bracketed the sample pH value and were

equilibrated to the sample temperature. Water

samples were filtered through a 142-millimeter

(mm)-diameter, 0.1-micrometer (m)-pore-size

filter for major, minor, and trace element

analyses. At the USGS Boulder, Colo. facility

concentrations of REEs, Zn, U, and Th were

determined by ICP-MS (Garbarino and Taylor,

1995) and concentrations of SO4were determined

by ion chromatography.

At a subset of sampling sites, 2- to 4-liters

of unfiltered, unacidified water were collected for


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the iron oxidation experiments. These samples

were stored at room temperature, and after 6

months, precipitates were concentrated and

filtrates were collected using tangential-flow

ultrafilters with a nominal cut-off of 10,000

molecular-weight. Filtrates were analyzed by

ICP-MS for selected major and trace elements.

Precipitates were digested following proceduresoutlined by Hayes (1993) and analyzed for major

and trace elements by ICP-MS. Mineral

identification was determined at Ohio State

University with X-ray diffraction.

Two well-characterized, acid mine water

samples were selected for new Standard

Reference Water Samples. Sample PPREE1 is

from the Paradise portal, upper Animas River

basin, Colorado, and sample SCREE1 is from

Spring Creek in the West Shasta mining district

of northern California. Fifty liters of each sample

were collected and filtration began within three

hours of collection in a USGS mobile laboratory

truck. Two parallel, 0.1-m, acid-cleaned, all-

plastic plate filters with 293-mm and 142-mm

diameters were used, and filtrates were

composited into a 50-L acid-washed carboy.

Filtration was completed within 1 hour, and the

pH of the reference waters was adjusted to less

than 2 with concentrated HNO3.

At the USGS laboratory in Boulder,

Colorado, each reference water was split into

250-milliliter aliquots using a ten position, Tefloncone splitter. The aliquots were capped, sealed

with parafilm, and numbered sequentially.

Seventeen participating laboratories were sent

two aliquots of each reference water with no

laboratory receiving sequential numbers. Samples

spanning the entire range of numbers were

analyzed to allow for recognition of sampling

biases.

RESULTS AND DISCUSSION

Standard Reference Water Samples

Seventeen international laboratories,

including four USGS facilities, participated in a

round-robin analysis to determine the most

probable values (MPVs) for the REEs (table 1).

MPVs were determined using a robust statistical

Table 1. Most probable values (MPV) with

median absolute deviation (MAD) for two

Standard Reference Water Samples. All values in

micrograms per liter.

PPREE1 SCREE1

Element MPV MAD MPV MAD

La 80.4 5.9 9.85 0.73Ce 161.2 7.7 24.6 2.2

Pr 21.2 1.3 4.29 0.28

Nd 92.3 5.7 22.1 0.9

Sm 20.3 1.5 6.71 0.31

Eu 5.95 0.48 1.47 0.07

Gd 23.8 1.7 8.21 0.65

Tb 3.65 0.33 1.34 0.07

Dy 22.0 0.7 8.10 0.34

Ho 4.43 0.09 1.61 0.06

Er 11.9 0.4 4.35 0.21

Tm 1.48 0.05 0.582 0.023

Yb 8.20 0.13 3.39 0.17

Lu 1.12 0.03 0.452 0.014

treatment that is insensitive to outliers (Peart and

others, 1998).

In general, there was good agreement in the

REE determinations among the participating

laboratories. For PPREE1 and SCREE1, 87 and

83 percent, respectively, of the individual

laboratories results overlap the MPVs. The

percent uncertainty for the individual REE

concentrations varies from 2 to 9 percent. The

REE reference waters are available upon request.

Identifying Source-Water Signatures

Within the upper Animas River watershed

in Colorado numerous naturally-occurring acid

springs and acid mine waters contribute metals to

the streams. One goal of the AML initiative is to

define the current baseline conditions in the

watersheds and differentiate between natural and

mining contributions of metals to the streams. A

number of different techniques are being assessed

to reach this goal, including identifing source-water signatures using REE geochemistry.

Two subbasins in the Animas basin with

different geological characteristics, including

bedrock composition and types of alteration and

mineralization, were chosen to investigate

techniques for identifying source-water

signatures. Prospect Gulch (fig. 1) lies within the


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Figure 1. Map of upper Animas River basin.MFMC = Middle Fork Mineral Creek, SFCC =

South Fork Cement Creek.

Silverton Caldera, and the bedrock consists

primarily of the Burns Formation, a volcanic unit

of rhyodactitic flows and tuffs. Alteration ranges

from propylitic in the southern part of the basin to

quartz-sericite-pyrite and quartz-alunite in the

northern part (Bove and others, 1998). The

second subbasin, Middle Fork Mineral Creek

(MFMC), is located west of the Silverton Caldera

and is underlain by the San Juan Formation,

thickly-bedded, reworked volcaniclastic deposits

that have been intruded by quartz monzonite

porphyries. The volcanic rocks surrounding the

largest porphyry are altered to varying degrees

from quartz-sericite pyrite to propylitic (Ringrose

and others, 1986).

In Prospect Gulch, five mine waters (pH 2.4

to 3.6) and four springs (pH 3.3 to 5.7) were

sampled during August and September 1997. The

REE patterns (fig. 2) display a middle REE

enrichment with a maximum at Eu or Gd.

Overall, the springs and the mines have similar

patterns with the exception of one spring sample,which has a negative Ce anomaly.

During September 1995 five mine waters

(pH 3.1 to 5.7) and five springs (pH 3.1 to 6.8)

were sampled in the MFMC. The REE patterns

display a greater range in shape than the samples

from Prospect Gulch. The mine waters have two

types of patterns (fig. 3), two samples display a

10-4

10-5

10-6

10-3

LaCePrSm NdEuGdTbDyHoErTmYbLu

CONCENTRATIO

N/NASC

Figure 2. Rare earth element diagram of watersfrom Prospect Gulch. Concentrations normalizedto NASC (Haskin and others, 1968; Gromet andothers, 1984). Triangles-mine waters, crosses-natural springs.

more sinusoidal pattern and three display a

middle REE enriched pattern. The two samples

with the sinusoidal pattern are from draining adits

on the north side of the basin, which is

predominantly underlain by propylitically altered

volcanic rocks, and the three middle REE

10-4

10-5

10-6

10-3


C

ONCENTRATION/NASC

Figure 3. Rare earth element diagram of waters

from Middle Fork Mineral Creek. Triangles-mine

waters, crosses-natural springs.


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enriched mine waters are from the south side of

the basin, within or near the quartz monzonite

porphyries. The REE patterns of the spring waters

display middle REE enrichment with four of the

five samples having negative Ce anomalies.

The presence of a negative Ce anomaly

most likely reflects differing redox conditions in

some of the spring environments as comparedwith the mine settings. Cerium anomalies have

been observed in shallow groundwater samples

from the Carnmenellis district, England and are

believed to be a result of the oxidation of Ce (III)

to insoluble Ce (IV), with subsequent removal

(Smedley, 1991). The loss of Ce relative to its

neighboring REEs, La and Pr, produces a

negative Ce anomaly in the REE patterns.

Because Ce anomalies were only observed in

some of the spring samples, Ce anomalies may

not prove to be a usable source signature.

Determinations of whole rock REE compositions

of the major geologic units within the subbasins

are underway. Preliminary data indicate that the

REE patterns of the waters, with the exception of

the negative Ce anomaly, reflect the REE

compositions of the lithologies along the flow

paths.

In the Boulder watershed (Montana) acid

mine waters from Crystal Mine and Bullion Mine

were analyzed to compare the REE patterns of the

Animas water samples with waters derived from

different geologic terrains. The two mines occuralong a mineralized structure within the Butte

Quartz Monzonite of the Boulder batholith

(Ruppel, 1963). The REE patterns (fig. 4) are

nearly flat with negative Eu anomalies. The host

monzonite is characterized by a relatively flat

REE pattern with a negative Eu anomaly (Lambe,

1981).

Because the REE patterns of the acid waters

seem to reflect the REE patterns of the host rocks,

a contrast in the REE composition of the host

rock is needed to enable use the REE patterns of

acid waters as a source signature. In the Animasbasin studies, the mineralization does not appear

to significantly affect the REE concentration of

the acid waters, such that comparing the REE

concentrations to other metals enriched in the

mineralized zones may distinguish mine waters

from natural springs. In the suite of samples from

MFMC and Prospect Gulch, for a given La

Bullion Mine (4.8)

Crystal Mine (3.1)

10-4

10-5

10-3


CONCENTRATION/NASC

Figure 4. Rare earth element diagram of twomine waters, Boulder basin, Mont. Value of pH inparentheses. Crystal Mine sampled at adit, BullionMine sampled in creek below dump pile.

concentration, the mine waters have distinctly

higher Zn concentrations compared to

background spring samples (fig. 5). Other

elements that are not enriched in the mineralized

areas may act similarly to La. This observation

may prove useful for differentiating between

mining and natural sources in areas where the

origin (natural or mining-influenced) of acidseeps is uncertain.

10

100

1000

10000

1001010.10.01

Zn

(MICROGRAMSPERLITER)

La (MICROGRAMS PER LITER)

Figure 5. Relation of dissolved La to Zn for mine(triangles) and spring (crosses) waters in ProspectGulch and Middle Fork Mineral Creek.


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In-Stream Processes

The fate and transport of REEs entering the

stream environment were investigated using field

and laboratory studies. A 2-km stream reach of

the South Fork Cement Creek (SFCC), Colo. was

sampled during low flow in October 1996.

Downstream the pH decreases from 7.1 to 5.1,and loading of REEs and major and trace

elements increase due to the addition of acid mine

drainage and acid springs. Comparing the

measured load at the lowermost sampling site to

the sum of the input loads accounts for 77 to 93

percent of the REEs. In contrast, measured loads

of Ca, Sr, SO4, Zn, and Co averaged 117 3

percent of the summed input loads. Values less

than 100 percent suggest that REEs are probably

being removed in this acidic, alpine stream. Loss

of REEs may be related to iron and aluminum

colloids that are actively precipitating in thestream channel.

To investigate fate and transport of REEs in

a stream reach where pH increases, a suite of

samples was collected from Uncle Sam Gulch in

the Boulder watershed, Mont. during July 1997.

In contrast to SFCC where a number of acid water

sources contribute metals to the creek, Uncle Sam

Gulch has only one dominant acid water source.

Acid mine water from the Crystal Mine (pH=3.1)

enters the stream, lowering the pH from 7.2 to

3.6. Within 2 km, the stream pH increases to 6.9,

apparently due to dilution or neutralization by

circumneutral waters. The streambed is coated

with iron precipitates throughout this reach.

Upstream of the lower most sampling site, the

iron stained stream bed is a lighter color than

below the mine site, suggesting that aluminum is

precipitating as well.

The total REE concentrations of the stream

water along this reach decrease from 31.2 g/L to

0.5 g/L. To evaluate if the reduction in the REE

concentrations is due to REE removal or due to

dilution, we compare the REE variation with aconservative solute, SO

4. Downstream from the

mine, the REE/SO4

ratio remains relatively

constant until the pH of the stream is above a

value of 4.3 (fig. 6) indicating that the REEs

behave conservatively through this pH range.

Compared to SO4, the REEs are removed from

solution before the next sample site, which has a

pH value of 6.9. A similar pattern is

0 500 1000 1500 2000

0.00

0.05

0.10

0.15

0.20

DISTANCE (METERS)

REE/SO4

5

10

15

20

25

303.1

3.63.9 4.3

6.9

Al/SO4

Figure 6. Dissolved REE/SO4(squares) and

Al/SO4(circles) for stream water samples, Uncle

Sam Gulch, Mont. Distance downstream fromCrystal Mine adit. Numbers above symbol = pHvalue of sample.

observed with Al, suggesting that the REEs may

have coprecipitated or adsorbed onto aluminum

colloids. With this limited data set, we are not

able to differentiate between the relative

importance of the increase in pH on the extent ofadsorption and the role colloid composition plays

on the removal of REEs and other metals.

During the August 1998, a tracer

experiment was carried out in Uncle Sam Gulch.

A subset of samples are being analyzed for REEs

and trace metals to better determine the roles of

colloid formation and pH variation on the

attenuation of REEs and other metals and to

quantify the mass transfer from solution to colloid

during transport.

REE Partitioning during LaboratoryIron Oxidation Experiment

A laboratory experiment was undertaken to

study the partitioning of the REEs between iron

colloids and aqueous solutions. These laboratory

results will provide a geochemical framework for

interpreting field data on fate and transport of


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REEs and other metals in acidic streams.

Unfiltered, unacidified mine water samples were

collected, and the Fe (III) allowed to oxidize at

room temperature for six months. The set of

waters had initial pH values ranging from 1.7 to

6.2, specific conductance from 775 to 17,000

microsiemens per centimeter, dissolved Fe

concentrations from 50 to 675,000 g/L, and Laconcentrations from 1 to 210 g/L. The dissolved

iron oxidized and precipitated during the 6-month

interval, and at the conclusion, water and

precipitates were separated and analyzed for

major and trace elements.

The precipitates are enriched in REEs

relative to their respective waters, with the

enrichment strongly dependent on pH (fig. 7).

The REE patterns of the filtrates and precipitates

are convex-up with enrichment in the middle

REEs relative to the light and heavy REEs. For a

given sample, the filtrates and their original

waters have similar REE patterns and

concentrations because less than 5 percent of the

REEs were removed from solution during the

experiment. These results indicate that during

iron colloid formation, the REEs may be removed

from solution without altering the REE pattern of

the solution and at pH values less than

approximately 4.5, most of the REEs stay in

solution.

REEs as Chemical Analogues forActinides

Rare earth elements have been used as

chemical analogues for actinides in a number of

biological and geological studies because of their

similarity to the actinides in ionic charge and

ionic radius (Weimer and others, 1980). We have

investigated the role of water composition and

colloids in the attenuation of U, Th, and REEs in

an acidic weathering environment, and the extent

to which the natural analogue concept is

appropriate. Within the upper Animas River

basin, naturally-occurring acid springs (pH 2.7-

6.8) and acid mine drainage (pH 2.3-6.4) dissolve

minerals in the country rock, releasing U, Th, and

REEs, which are predominantly derived from

apatite, monazite, and epidote. Upon entering the

stream system, concentrations of trace elements

can be attenuated by adsorption to colloids or

1 2 3 4 5

1

10

100

1000

10000

100000

1000000

PRECIPITATECONCENTRATION/AQUEOUSCONCENTRATION

Goethite Schwert.Jarosite Schwert.

SL

PPIM1 IM2 CACM

Th

U

REE

pHf

Figure 7. Relation of solid phase enrichment to

pH. Concentration in precipitate (g/g) relative to

aqueous phase (g/g) from iron oxidation

experiments. Sample designation: IM1 - IronMountain site, Calif. (pH

i= 1.6, pH

f= 1.7), IM2 -

Iron Mountain site, Calif. (pHi= unknown, pH

f=

2.0), CA - Chandler adit, Colo. (pHi= 2.6, pH

f=

2.5), CM - Crystal Mine, Mont. (pHi= 3.1, pH

f=

2.7), PP - Paradise portal, Colo. (pHi= 5.3, pH

f=

3.4), and SL - Silver Ledge Mine, Colo. (pHi= 6.1,

pHf= 4.2). pH

i= pH initial, pH

f= pH final.

Dominant mineral phase of precipitate shownbelow sample. Schwert. = schwertmannite.

coprecipitating. Laboratory iron oxidation

experiments on mine waters, described above,

were run to determine the partitioning of the U,

Th, and REEs over a range of pH conditions.

In the Animas water samples U, Th, and the

REE concentrations are inversely correlated with

pH (fig. 8); however, the slopes are strikingly

different. The REE show a gradual decrease in

concentrations with increasing pH, where as U

and Th concentrations have large decreases

between pH 3 and 4, then remain relatively low

and constant above a pH of 4. Results from the

laboratory experiments show that the precipitates

become increasingly enriched in U, Th, and REEs

compared to the aqueous phase with increasing


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2 3 4 5 6 710

-4

10-3

10-2

10-1

100

101

102

U and Th variation

REE variation Th

U

REE

CONC

ENTRATION

(MICROGRAMSPERLITER)

pH

Figure 8. Relation of dissolved U, Th, and REE

vs. pH for upper Animas River basin samples,including natural springs, mine effluents, andstream waters.

pH (fig. 7). With the exception of the pHi = 5.3sample that was predominately goethite,

compositions of the solid phases vary from

dominantly jarosite in the low-pH samples to

dominantly schwertmannite in the high-pH

samples. Aluminum-rich phases were not

observed in any of the precipitates.

The field and laboratory results suggest that

U and Th are primarily adsorbed by hydrated

ferric oxides at pH values of 3 to 4. In contrast,

REEs tend to remain dissolved until higher pH

values are reached, in a manner similar to Al.

Aluminum remains in solution until the pH

reaches about 5, then hydrolyzes and precipitates

as a hydroxysulfate mineral (Nordstrom and Ball,

1986). REEs will likely coprecipitate or adsorb

with aluminum-rich solids.

CONCLUSIONS

The AML pilot watersheds are well-suited

for investigating the REE geochemistry of acid

waters because of the numerous acid water

sources and the interdisciplinary approach towatershed characterization. Using REE patterns

as a tool to identify source signatures of acid

waters is valuable when the REE patterns of the

lithologies along the flow path have different

REE patterns. To differentiate between natural

springs and mine waters using only the REE

patterns, either the mineral deposits must have a

REE signature distinct from the surrounding

lithologies, or secondary processes in the mining

or spring environment must lead to REE

fractionations. Within the subbasins of the upper

Animas watershed, using REE patterns to

differentiate between acid springs and mine

waters may not be conclusive. Although many

acid springs have REE patterns with negative Ceanomalies and the mine waters do not, not all the

springs sampled have such patterns.

Within the Animas River basin, the mine

environment does not appear to enrich the acid

drainage in REEs; thus, comparing the REE

concentrations to other metals enriched in the

mine waters, such as Zn, may provide a means to

discriminate between mining and natural acid

water sources. This differential enrichment should

provide a useful tool, in conjunction with other

geochemical indicators, for determining if seeps

in areas impacted by mining are natural or

mining-related.

Formation of Fe and Al colloids plays a

role in the attenuation of REEs and other metals

in streams that receive acid waters. Field and

laboratory experiments demonstrate that REEs are

removed from solution at pH values greater than

4.5 and that only minor fractionations occur.

ACKNOWLEDGEMENTS

The work by Philip Verplanck was fundedby the National Research Councils post-doctoral

research program and the USGS AML initiative.

B. McCleskey, T. Brinton, D. Roth, and R.

Antweiler provided analytical support. Numerous

scientists working in the two watersheds provided

the framework for this study. Reviews by J. Ball

and A. Mast greatly improved the manuscript.

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AUTHOR INFORMATION

Philip L. Verplanck, D. Kirk Nordstrom, and

Howard E. Taylor, U.S. Geological Survey,

Boulder, Colorado

overview of rare earth elements investigation in acid waters of us geological survey abandoned mine...

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